1. Field of the Invention
The present invention generally relates to a froth flotation deinking process for use in paper recycling. More particularly, the present invention pertains to a process for the froth flotation separation of paper and wood fibers from ash, fillers, printed inks and other contaminants during wastepaper recycling. A liquid solution containing a frothing agent is mechanically added prior to, or during, the separation process to the upper surface or portion of a wastepaper pulp slurry which has been introduced into a froth flotation deinking device (or to the upper surface or portion of a froth layer produced in the device), rather than being added to, or mixed with, the wastepaper pulp slurry before the slurry is introduced into the froth flotation deinking device.
2. Background and Description of Related Art
The recycling of wastepaper is of growing importance in the protection of the environment by reducing the amount of solid wastepaper and sludge which is placed into landfills. Wastepaper is the largest contributor of the solid waste landfilled each year. In 1988, wastepaper made up about 40% of the municipal solid waste in the United States. About 52 million tons of wastepaper are currently landfilled and, thus, pollute the environment. Further, currently-employed wastepaper recycling processes produce about 0.1 ton of waste dry sludge per 1 ton of dry paper fibers recovered from the recycling process. The untreated sludge contains other environmental pollutants, such as various chemicals, mineral fillers and wood fibers.
Although the paper recycling rate has increased in recent years, the quality of various grades of paper made from recycled wastepaper fibers is much poorer than the quality of similar grades of paper made with virgin (not recycled) paper fibers. Further, the cost of making paper from recycled paper fibers is significantly greater than the cost of making paper from virgin paper fibers.
The objective of paper recycling is to recover paper fibers from wastepaper, such as paper photocopied on a Xerox.RTM. or other brand photocopier, which may contain ash, various chemicals, printed or copied inks (offset ink, copying toner particles, etc.) and/or other contaminants.
Froth flotation is an important technique used in the recycling of wastepaper for removing ink and other contaminants from the wastepaper. This technique has been used to effect the separation of various materials from one another, such as a mineral from its ore, for many years.
Froth flotation employs the principles of colloid chemistry, crystallography and physics to separate floatable (hydrophobic) and non-floatable (hydrophilic) particles from each other in an aqueous slurry containing such materials based upon differences in the hydrophobicity of the materials.
Generally, a slurry made from the materials to be separated and water, which has already been thoroughly mixed with flotation reagents (cationic or anionic collectors), conditioning reagents and/or frothing agents (surfactants, frothers or dispersants), is introduced into a froth flotation device, and a pressurized nonreactive gas, such as air, is introduced into the bottom of the device by a generator, and is forced upwards into the slurry in the form of bubbles ranging generally in size from about 50 microns to about 2 or 3 mm in diameter. The air bubbles tend to attach to the floatable particles present in the slurry, and cause those particles to rise upwards to the surface of the slurry as a froth layer. The bubble carrying capacity is largely a function of the surface area of the bubbles per unit volume of the froth. Ideally, only the hydrophobic particles should "float" (adhere to the air bubbles) and rise upwards to the surface of the slurry. However, due to a mechanism known as "entrainment," a considerable amount of hydrophilic particles, such as fibers, and process water (water used to make the pulp slurry), also rise to this surface and become entrained into the froth phase along with the bubbles. To eliminate this entrainment problem and, thereby, improve the separation process, wash water may be introduced into the top of the froth flotation device and distributed onto, or in, the froth layer to scrub entrained non-floatable particles from the froth layer. The wash water descending through the froth flotation device induces entrained non-floatable particles to separate from the froth layer and drop by gravity (sink) through the device. The washed froth layer overflows from the top of the froth flotation device, leaving behind the non-floatable particles. The fraction containing the non-floatable particles is withdrawn from the bottom of the device by gravity or by a pump. However, the foam may be destroyed by the washing water.
In the past, flotation, conditioning and frothing reagents have been added to, and directly mixed with, the pulp slurry prior to the introduction of the slurry into the froth flotation device in order to distribute these reagents on the surface of the particles targeted to be removed by flotation. See, for example, U.S. Pat. Nos. 4,592,834, 4,804,460, 4,981,582, 4,997,549, 5,073,253, 5,116,487, 5,167,798 and 5,397,001, which relate to the froth flotation separation of a mineral from its ore.
Froth flotation deinking (ink removal) processes involve interactions among air bubbles, ink particles and wastepaper and/or wood fibers, and typically have three subprocesses: (a) detachment of the ink particles from the wastepaper or wood fibers; (b) adhesion of the ink particles onto air bubble surfaces; and (c) removal of froth and ink particles from flotation cells. The ink particles, many of which, such as offset ink and copying toner particles, are hydrophobic, attach to the surface of the air bubbles and float upwards with the bubbles towards the upper portion of a froth flotation device during flotation. Under ideal conditions, the hydrophilic paper fibers will not attach to a hydrophobic air bubble surface and, thus, will not float during the flotation process. Thus, fiber loss should not occur. However, fiber loss (and water loss) is a significant problem in many paper recycling mills. A detailed description of the chemistry involved in the froth flotation deinking of wastepaper is provided in L. D. Ferguson, "Deinking Chemistry: part 1," Tappi J, 75(7), Page 75 (1992) and L. D. Ferguson, "Deinking Chemistry: Part 2," Tappi J, 75(8), Page 49 (1992).
The true flotation (adhesion flotation) and entrapment of wastepaper and wood fibers in froth flotation deinking processes may be determined by measuring fiber removal and water removal by standard methods at different froth heights. The intercept obtained by plotting the fiber removal against water loss obtained at different froth heights represents the true flotation, and the slope of the same curve represents the fiber entrapment by froth network.
Deinking quality and efficiency is often measured by the brightness (or brightness gain) and whiteness, and dirt counts, of handsheets made from the paper fibers deinked during the froth flotation process.
In froth flotation deinking, a chemical surfactant may act as: (a) a dispersant to separate ink particles from the surface of wastepaper or wood fibers and to prevent the redeposition of separated ink particles on the fibers; (b) a collector to agglomerate small particles to large ones, and to change the surface of ink particles which are hydrophilic to hydrophobic; and (c) a frother to generate a layer of foam at the upper portion of the froth flotation device for removal of ink particles and other contaminants. A collector can also cause a surface energy change between solid-liquid, solid-gas, and gas-liquid interfaces. However, a collector is generally not considered to be a surfactant. While it is generally not necessary to use a dispersant or collector for the froth flotation deinking of wastepapers, a frothing agent generally must be used.
Many problems occur during conventional froth flotation processes for the deinking of wastepaper. The addition of a frothing agent directly to the pulp slurry to be introduced into a froth flotation device for separating ink and other contaminants from wastepaper and wood fibers causes several adverse effects.
The paper fibers become contaminated by the adsorption of the frothing agent onto the fibers and, as a result, have a diminished fiber-fiber bonding during papermaking, with the quality of paper being made from these contaminated paper fibers being poor. Moreover, the frothing agent disadvantageously causes foaming to occur on paper-making machines.
Further, the hydrophobicity of the surfaces of the ink particles and, thus, the adhesion of the ink particles to the air bubbles, become reduced if frothing agents are adsorbed onto ink and air bubble surfaces. This results in a decrease in the ink removal efficiency of the process.
Additionally, a high level of paper fiber loss occurs during the froth flotation separation of the ink and other contaminants from the wastepaper and wood fibers. Such loss is caused by the physical entrapment of the fibers in the air bubble network which rises towards the top of the froth flotation device to the froth layer, and by the adhesion of hydrophobic portions of the surfaces of the fibers on the surfaces of the air bubbles present within the device. Fiber losses of from 4 to 24 weight percent have been reported, depending upon the conditions and equipment employed in the froth flotation deinking process. Such fiber loss significantly decreases paper recycling productivity, and significantly increases the costs of paper recycling. It is estimated that a 5% increase in the recovery of paper fibers during a froth flotation deinking process may significantly increase paper recycling productivity, may significantly reduce the costs of paper recycling, and may reduce the dry sludge production in a typical paper recycling mill having a capacity to recycle 250 tons of wastepaper per day by about 2 tons per day (by about 700 tons per year). A 10% increase in the paper recycling rate results in a reduction of 8.8 million tons of wastepaper in landfills each year. The mechanisms of pulp loss during froth flotation deinking are described in Ajersch et al., "Mechanisms of Pulp Loss in Flotation Deinking," J. Pulp and Paper Sci. 22, 9:J338-345 (1996).
Water loss also occurs during conventional froth flotation deinking processes as a result of process water (water used to make the pulp slurry) rising upwards to the top surface of the slurry and becoming entrained in the froth layer. Water loss caused by froth entrainment in froth flotation deinking is generally approximately 15 to 30 tons per ton of dry paper. Such water loss also significantly reduces paper recycling productivity, and significantly increases the costs of paper recycling.
It would be advantageous to provide a method for the froth flotation separation of ink and other contaminants from wastepaper and wood fibers which reduces fiber loss, reduces water loss, reduces the amount of frothing agent required to be used during the process (and thereby reduces or completely eliminates the contamination of the paper fibers by frothing agent), and increases the efficiency of the deinking process.
The froth flotation separation processes of the present invention achieve all of the advantages of conventional froth flotation deinking processes. These processes also achieve the additional advantages described in the preceding paragraph, by mechanically applying a liquid solution containing a frothing agent to the upper surface or portion of the pulp slurry introduced into a froth flotation deinking column (or to the upper surface or upper portion of the froth layer produced therein), either prior to, or during, the froth flotation process, preferably by spraying the solution from the top of the froth flotation device onto the upper surface of the pulp suspension (or froth phase) during the froth flotation deinking process, rather than by directly mixing the frothing agent with the pulp slurry prior to introducing the pulp slurry into the froth flotation device.